Voltage or battery level on display?

I’m building a new test box, and I would like to include an oled to display battery level or battery voltage. Is there a simple way to do this? Ideally I’d like actual voltage, but I’d settle for a battery percentage or icon. for this application I’m looking more for functionality than user friendliness.

It would need to be coded, but wouldn’t be particularly difficult since battery measurement and character rendering already exists.